Optical networks using dense wavelength-division multiplexing (DWDM) technology is a key solution to keep up with the tremendous growth in data traffic demand. However, as WDM transmission technology matures, the ability to manage traffic (including switched and protected traffic) in a WDM network is becoming increasingly critical and complicated. In particular, the rapid advances in dense WDM technology (with hundreds of wavelengths per fiber) and world-wide fiber deployment have brought about a tremendous increase in the size (i.e., number of ports) of optical cross-connects (OXCs), as well as the cost and difficulty associated with controlling such large OXCs. In fact, despite the remarkable technological advances in building photonic cross-connect systems and associated switch fabrics, the high cost (both capital and operating expenditures) and unproven reliability of huge switches (e.g., with 1000 ports or more) have not justified their deployment.
Waveband Switching (WBS) in conjunction with multi-granular OXC (MG-OXC) architectures has been proposed to support the ever-increasing traffic while maintaining the cost and complexity of the system at a reasonable level. The main idea of WBS is to group several wavelengths together as a band, and switch the band as a single entity (i.e., using a single port) whenever possible (that is a band is demultiplexed into individual wavelengths if and only if necessary, e.g., when the band carries at least one lightpath which needs to be dropped or added). A complementary hardware is MG-OXC that not only can switch traffic at multiple levels such as fiber, wavelength band (or waveband), and individual wavelength (or even sub-wavelength), but also can add and drop traffic at multiple levels, as well as multiplex and demultiplex traffic from one level to another within an MG-OXC itself. By using WBS in conjunction with MG-OXCs, the total number of ports required in such network (to be called a WBS network hereafter) to support a given amount of traffic can be much lower than that in a traditional wavelength routed network (WRN) that uses ordinary OXCs (that switch traffic only at the wavelength level). The reason is that 60–80% of traffic simply bypasses the nodes, and hence the wavelengths carrying such transit traffic do not need to be individually switched in WBS networks (as opposed to WRNs wherein every such wavelength still has to be switched using a single port).
In addition to reducing the port count (which is a major factor contributing to the overall cost of switching fabrics), the use of bands reduces the number of entities that have to be managed in the system, and enables hierarchical and independent management of the information relevant to wavebands and wavelengths. This translates into reduced size (footprint), power consumption and simplified network management. Moreover, relatively small-scale modular switching matrices are now sufficient to construct large-capacity optical cross-connects, making the system more scalable. With WBS, some or most of the wavelength paths (or lightpaths) do not have to pass through individual wavelength filters, thus simplifying the multiplexer and demultiplexer design as well. In fact, cascading of FTB and BTW demultiplexers has been shown to be effective in reducing cross-talk, which is critically important in building large capacity backbone networks. Finally, all of these also result in reduced complexity of controlling the switch matrix, provisioning and providing protection/restoration in a similar way.
This patent describes efficient heuristic algorithms for WBS with MG-OXCs. This work considers the case of both off-line and on-line traffic. Optimal Integer Linear Programming or ILP formulations and efficient heuristic algorithms are provided for the off-line MG-OXC network design and dimensioning. For the on-line traffic, efficient heuristic algorithms are provided to reduce the used number of ports or alternately minimize the blocking to the traffic for a given number of ports. The work also provides methodologies for efficient protection and restoration with wavelength and waveband conversion in MG-OXC networks. This technology can be applied to consider various survivability schemes with SRLG and also to networks with limited or no wavelength conversion capabilities. In addition, the technology can also be applied to hybrid optical networks, i.e., networks consisting of a mix of both ordinary OXC and MG-OXCs. A new cost effective MG-OXC architecture and cost model is also provided.
Other Solutions
In this section we review some of the existing work on MG-OXC and WBS. So far, only very limited research has been done on MG-OXC and WBS. The concept of WBS was researched in rings and on MG-OXCs in mesh networks, a couple of IETF drafts on Generalized Multi-protocol Label Switching (G-MPLS) control plan extension to WBS networks, and one journal paper on WBS algorithms.
The other research discusses how optical bypass can be efficiently realized using wavelength bands in rings (LANs or MANs). The feasibility and cascadability of MG-OXCs in rings were investigated either via computer simulation or prototyping. Limited analytic work for some special traffic patterns in rings is done. However, none of these works addressed WBS in networks with the mesh topology (useful for the WANs). Others have suggested a two-layer switching fabric containing a band cross-connect (BXC) and a wavelength cross-connect (WXC). Others added a new switching layer, i.e., a fiber cross-connect (FXC) (but without wavelength conversion or waveband conversion capabilities), still others have proposed a single layer MG-OXC (which also does not include wavelength conversion or waveband conversion). In addition, a waveband OXC structure with “broadcast and select” function was proposed. None of these papers offered any interesting WBS algorithms.
Wavelength Grouping for WBS
There are several wavelength grouping strategies including: (1) end-to-end grouping: grouping the traffic (lightpaths) with same source-destination (s-d) only; (2) one-end-grouping: grouping the traffic between the same source (or destination) nodes and different destination (or source) nodes; (3) sub path grouping: grouping traffic with common sub path (from any source to any destination). Note that all existing work assumes either Strategy 1 or 2. Strategy 3 as it is the most powerful (in terms of being able to maximize the benefits of WBS) although it is also the most complex to use in WBS algorithms. Others studied a two-layer MG-OXC as in assuming wavelength grouping Strategy 2 (with one-end-grouping) only (which makes it difficult to take advantage of the benefits of WBS), and full wavelength conversion.
MG-OXC Architectures
Prior research typically considered MG-OXC architectures as shown in FIGS. 1 and 2. Note that these architectures themselves are our novel extension (i.e., with wavelength and waveband conversion banks, Tx/Rx and DXC components) to existing architectures. The first MG-OXC architecture includes the FXC, BXC and WXC layers. As shown in the Figures, the WXC and BXC layers consist of cross-connect(s) and multiplexer(s)/demultiplexer(s). The WXC layer includes a wavelength cross-connect (WXC) switch that is used to bypass/add/drop lightpaths at this layer, band-to-wavelength (BTW) demultiplexers, and wavelength-to-band (WTB) multiplexers. The BTW demultiplexers are used to demultiplex bands into wavelengths, while the WTB multiplexers are used to multiplex wavelengths into bands. At the BXC layer, the waveband cross-connect (BXC) is used to switch wavebands. The BXC layer also includes the fiber-to-band (FTB) demultiplexers and band-to-fiber (BTF) multiplexers. Similarly, fiber cross-connect (FXC) is used to switch fibers at the FXC layer. This architecture is dynamic in that (1) which fiber(s) and which band(s) in the fiber(s) to go through the FTB and BTW demultiplexers, respectively, can be dynamically reconfigured; and (2) some waveband(s) and some wavelength(s) can go through the waveband conversion and wavelength conversion, respectively. Clearly, this architecture is the most flexible as it allows a completely dynamic reconfiguration of the fibers, bands, and wavelengths for drop, add or bypass.
Compared to the first MG-OXC, the second one is a single-layer MG-OXC which has only one common switching fabric, as shown in FIG. 2. This switching matrix includes three logical parts corresponding to FXC, BXC and WXC, respectively. However, the major differences are the elimination of FTB/BTW demultiplexers and BTF/WTB multiplexers between different layers, which results in a simpler architecture to implement, configure and control. Another advantage of this single-layer MG-OXC is better signal quality because all lightpaths that do not require wavelength or waveband conversion go through one switching fabric (except those requiring conversion) where as in the multi-layer MG-OXCs, some of them may go through 2 to 3 switching fabrics (i.e., FXC, BXC and WXC). As a tradeoff, some incoming fibers, e.g., fiber n, are pre-configured as “designated fibers”. Only designated fiber(s) can have some of its bands dropped while the remaining bands bypass the node, all other non-designated incoming fibers (e.g., fibers 1 and 2) have to have all the bands either bypass the node entirely or be dropped entirely. Similarly, within these designated fiber(s), only designated band(s) can have some of its wavelengths dropped while the remaining bands bypass the node.
Each of these architectures are either limited or otherwise extreme solutions. For example, the first one may be an overkill and hence too expensive, complicated and unnecessary, while the second may be too limited in terms of its adaptability (reconfigurability) to efficiently reduce the port count.
There has been very limited work on efficient optical network design using MG-OXC with WBS. They all represent early stage work that is neither comprehensive nor complete. In fact, many basic problems including which MG-OXC architectures should be used, and how wavelength grouping can be done efficiently in WBS networks are still open. More advanced issues such as survivability using novel waveband recovery schemes and wavelength, waveband conversion in MG-OXC networks, though important have not yet been studied.
All prior work either assumed only simple metro-area or ring networks. Accordingly, very few simple and inefficient ILP formulations and heuristics for wavelength grouping were developed for WBS. In particular most work considered restricted simple wavelength grouping techniques, such as grouping traffic from the same source(s) to same destination(s)
or trying to band or group lightpaths with the same destination(s) only. In addition, these works simplify the problem by assuming full wavelength conversion capability at all nodes, which may not be the case in reality.
Further, all prior work has considered WBS for static off-line traffic only. None of the work has considered WBS and MG-OXC architectures and design for dynamic on-line traffic.
The new challenges in designing WBS networks require innovative solutions that can only be obtained by building upon and advancing the knowledge of, and techniques for WRNs. More specifically, although a tremendous amount of work on WRNs has been carried out, and wavelength routing is still fundamental to a WBS network, the work on WBS (and MG-OXCs) in terms of the objective and techniques are quite different from all existing work on WRNs. For example, a common objective in designing (dimensioning) a WRN is to reduce the number of wavelengths required or the number of wavelength-hops used (which is a weighted sum taking into account the number of hops a wavelength path spans).
Due to possible failures of the ports and multiplexers/demultiplexers within a MG-OXC that are dedicated to wavebands, as well as possible failure of waveband converters, one or more wavelength bands in one or more fibers may be affected, but not the entire fiber or link (cable). Existing protection restoration approaches deal only with failures of individual wavelengths and fiber/link failure. Hence, new approaches and techniques to provide effective protection and restoration based on the novel concept of band-segment become interesting, so does the novel use of waveband conversion and/or wavelength conversion to recover from waveband failures.
Open Issues in WBS with MG-OXC
To summarize, (1) none of the prior work has considered the issue of survivability and (2) waveband, wavelength conversion in MG-OXC networks. All prior work assumes full wavelength conversion capability at all the nodes in the MG-OXC network. (3) Further, the topic of efficient WBS under on-line dynamic traffic conditions has not been addressed. All existing work assumes that traffic is off-line i.e., given a priori and even so only develop simple grouping algorithms for WBS. (4) So far, existing work on WBS networks has focused on minimizing the port count only. Clearly, the cost of a multi-layer MG-OXC may be more (e.g., include additional FTB demultiplexers for interconnecting FXC and BXC layers). With respect to a network, we should consider not only the cost of all nodes, but also the cost of wavelengths/fibers (including amplifiers). (5) No research has focused on how to design MG-OXCs to cut down the overall cost of the system by not only reducing the number of ports, but also decreasing/simplifying other components such as multiplexers/demultiplexers and transmitters/receivers, as well as increasing bandwidth utilization (or reducing the number of wavelengths needed).